Taurine can be referred to as 2-aminoethanesulfonic acid and is one of the amino sulfonic acids found in the tissues of many animals. Taurine is an extremely useful compound because it has such pharmacological effects as detoxification effect, fatigue-relieving effect and nourishing and tonifying effect. As a result, taurine finds wide applications as an essential ingredient for human and animal nutrition.
Taurine is currently produced in an amount of over 50,000 tons per year from ethylene oxide and monoethanolamine. At present time, most of the taurine is produced from ethylene oxide, following a three-step process: (1) the addition reaction of ethylene oxide with sodium bisulfite to yield sodium isethionate; (2) the ammonolysis of sodium isethionate to yield sodium taurinate; (3) the neutralization with an acid, i.e., hydrochloric acid and, preferably, sulfuric acid, to generate taurine and inorganic salts.
Although the ethylene oxide process is well established and widely practiced in commercial production, the overall yield is not very high, less than 80%. Moreover, the process generates a large amount of waste stream that is increasingly difficult to dispose of.
The first stage of the ethylene oxide process, the addition reaction of ethylene oxide with sodium bisulfite, is known to yield sodium isethionate in high yield, practically quantitative, as disclosed in U.S. Pat. No. 2,820,818 under described conditions.
Therefore, the problems encountered in the production of taurine from the ethylene oxide process arise from the ammonolysis of sodium isethionate and from the separation of taurine from sodium sulfate.
According to the co-pending application U.S. Ser. No. 13/999,203 and U.S. Ser. No. 13/999,439, these same issues are also encountered in the production of taurine from ethanol and ethylene, respectively. Sodium isethionate is a key common intermediate and the ammonolysis of sodium isethionate is an important step in the ethanol and ethylene processes.
U.S. Pat. No. 1,932,907 discloses that sodium taurinate is obtained in a yield of 80%, when sodium isethionate undergoes ammonolysis reaction in a molar ratio of 1:6.8 for 2 hrs at 240 to 250° C. U.S. Pat. No. 1,999,614 describes the use of catalysts, i.e., sodium sulfate, sodium sulfite, and sodium carbonate, in the ammonolysis reaction. A mixture of sodium taurinate and sodium ditaurinate is obtained in a yield as high as 97%. However, the percentage for sodium taurinate and sodium ditaurinate in the mixture is not specified.
DD 219 023 describes detailed results on the product distribution of the ammonolysis reaction of sodium isethionate. When sodium isethionate undergoes the ammonolysis reaction with 25% aqueous ammonia in a molar ratio of 1:9 at about 280° C. for 45 minutes in the presence of sodium sulfate and sodium hydroxide as catalyst, the reaction products comprise 71% of sodium taurinate and 29% of sodium di- and tri-taurinate.
WO 01/77071 is directed to a process for the preparation of ditaurine by heating an aqueous solution of sodium taurinate at a temperature of 210° C. in the presence of a reaction medium. A mixture of sodium taurinate and sodium ditaurinate is obtained.
From these prior arts, it is therefore concluded that the ammonolysis of sodium isethionate invariably yields a mixture of sodium taurinate, sodium ditaurinate, and sodium tritaurinate. The percentage yield of sodium taurinate has not been more than 80%.
In order to obtain taurine from sodium taurinate, U.S. Pat. No. 2,693,488 discloses a method of using ion exchange resins, first strongly acid ion exchange resin in hydrogen form, and then an anion exchange resin in basic form. This process is complicated and requires the use of large quantity of acid and base to regenerate the ion exchange resins in each production cycle.
On the other hand, CN101508657, CN101508658, CN101508659, and CN101486669 describe a method of using sulfuric acid to neutralize sodium taurinate to obtain a solution of taurine and sodium sulfate. Crude taurine is easily obtained by filtration from a crystalline suspension of taurine after cooling. However, the waste mother liquor still contains taurine, sodium sulfate, and other unspecified organic impurities. It is desirable to have available a process for further separation of these components to achieve an economical process and to reduce the amount of waste stream.
It is, therefore, an object of the present invention to disclose a cyclic process for the production of taurine from alkali isethionate and from alkali vinyl sulfonate in a high overall yield, i.e., greater than 90%, in particular, greater than 95%. According to the process in the present invention, sodium ditaurinate and sodium tritaurinate, byproducts from the ammonolysis of sodium isethionate or sodium vinyl sulfonate, are continuously converted to sodium taurinate in the ammonolysis stage.
It is another object of the present invention to disclose a process for the preparation of pure sodium ditaurinate and pure sodium tritaurinate, and their conversion to sodium taurinate.
When sodium ditaurinate and sodium tritaurinate are reacted with aqueous ammonia under ammonolysis reaction conditions, a mixture of similar compositions of sodium taurinate, ditaurinate, and tritaurinate is formed in an equilibrium state. This novel finding renders the cyclic process possible.
It is a further object of the present invention to disclose a process for the effective separation of sodium sulfate from residual taurine, byproducts, i.e., sodium ditaurinate and sodium tritaurinate, and unreacted starting material, i.e., sodium isethionate. According to the process in the present invention, the residual taurine, which is less soluble, is converted to a highly soluble form, i.e., sodium taurinate or ammonium taurinate, to facilitate the cooling crystallization of sodium sulfate. The mother liquor, consisting of sodium taurinate, sodium ditaurinate, sodium tritaurinate, and sodium isethionate, is recycled to the ammonolysis reaction to produce sodium taurinate.